The present invention relates to a booster and a master cylinder for use in a brake system of a vehicle.
Examples of conventional electric boosters include the electric booster disclosed in Japanese Patent Public Disclosure No. 2007-191133. This electric booster comprises an axial member adapted to move forward and backward in response to an operation of a brake pedal, a cylindrical member disposed around the axial member so as to be movable relative to the axial member, and an electric actuator operable to cause a forward/backward movement of the cylindrical member. In this electric booster, the axial member and the cylindrical member face a primary pressure chamber of a tandem master cylinder, and a brake fluid pressure is generated in the tandem master cylinder by an input thrust force provided from the brake pedal to the axial member, and a booster thrust force provided from the electric actuator to the cylindrical member.
However, in the electric booster disclosed in Japanese Patent Public Disclosure No. 2007-191133, when the axial member moves forward under circumstances in which the fluid pressure in the primary pressure chamber of the tandem master cylinder is difficult to be increased for some reason, for example, bubble presence in the fluid, the fluid pressure in the primary pressure chamber remains low while the fluid pressure in the secondary pressure chamber is increased by a forward movement of the cylindrical member. As a result, a fluid pressure reactive force cannot be transmitted to the axial member (brake pedal). In this case, a driver continues the pedal operation, with a still weak pedal pressing force, i.e., without feeling a reactive force of the brake fluid pressure, whereby it becomes difficult to control the fluid pressure in the secondary pressure chamber.